U.S. Pat. No. 5,220,111 by Bucci et al, teaches a thermal treatment process for the stabilization of heavy metals-containing fly ash obtained by subjecting a flue gas containing acid gas components to scrubbing with an aqueous slurry of a calcium-containing compound and to particulate separation to recover the fly ash. The thermal treatment process comprises heating the calcium scrubbed fly ash to a temperature in the range from about 375.degree. C. to about 650.degree. C. and maintaining said temperature for a period of time from about 170 seconds to about five hours while in the presence of an oxygen containing gas to produce a thermally treated fly ash and a purge gas.
There ape several problems, however, relating to the temperature range in Bucci as follows:
(1) At temperatures below 400.degree. C., complete mercury recovery of any mercury which may be contained in the fly ash is not possible. Elemental mercury will not volatize below 357.degree. C. and mercury(I) chloride, suspected to form in reactions between mercury and halogen gases, will remain a solid below 400.degree. C. Additionally, dioxins ape only partially destroyed at temperatures below 400.degree. C.
(2) At temperatures above 600.degree. C., elemental arsenic and selenium will volatize and subsequently contaminate the purge stream from the thermal treatment process.
It is an object of the present invention to overcome these problems by narrowing the temperature range in Bucci.